Pour frozen spinach in saucepan and cover with water. Place uncovered saucepan on stove top and bring to a boil. Boil for 3 minutes. Reduce heat, cover and simmer for 4 minutes. Drain. Place oil in a skillet. Place on stove top and heat on high until hot. Add mushrooms. Constantly stirring, saute mushrooms for 4 minutes. Drain. Add spinach, garlic powder and salt. Over medium heat, cook mixture until thoroughly heated.